They are both good rounds. The .357sig was made to duplicate the .357mag 125 grain load. The .40S&W really shines with 165 grain - 180 grain loads. I carry 180 grain golddots in both my .40's.

.357sig can be hard to find ammo forvs. the .40S&W and it is ually a bit more. Wally world carries the value packs of .40S&W for about the same price as a regular box of .357sig (100 rounds vs. 50 rounds)

The .40S&W is more popular since most L.E.O. carry the round which helps in our ammo selection.

Like I already said, they are both good rounds. Buy some ammo and hit the range and see which one you like best. The .357sig usally has more muzzle flip which bothers some people.